U.S. patent application number 16/921096 was filed with the patent office on 2020-11-12 for method and apparatus for control of optical phase shifters in an optical device.
The applicant listed for this patent is Huawel Technologies Co., Ltd.. Invention is credited to Eric Bernier, Dritan Celo, Dominic John Goodwill, Chunhui Zhang.
Application Number | 20200355980 16/921096 |
Document ID | / |
Family ID | 1000005000584 |
Filed Date | 2020-11-12 |
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United States Patent
Application |
20200355980 |
Kind Code |
A1 |
Celo; Dritan ; et
al. |
November 12, 2020 |
Method and Apparatus for Control of Optical Phase Shifters in an
Optical Device
Abstract
A method and apparatus is provided for control of plural optical
phase shifters in an optical device, such as a Mach-Zehnder
Interferometer switch. Drive signal magnitude is set using a level
setting input and is used for operating both phase shifters, which
may have similar characteristics due to co-location and
co-manufacture. A device state control signal selects which of the
phase shifters receives the drive signal. One or more switches may
be used to route the drive signal to the selected phase shifter.
Separate level control circuits and state control circuits
operating at different speeds may be employed. When the phase
shifters are asymmetrically conducting (e.g. carrier injection)
phase shifters, a bi-polar drive circuit can be employed. In this
case, the phase shifters can be connected in reverse-parallel, and
the drive signal polarity can be switchably reversed in order to
drive a selected one of the phase shifters.
Inventors: |
Celo; Dritan; (Ottawa,
CA) ; Zhang; Chunhui; (Wuhan, CN) ; Goodwill;
Dominic John; (Ottawa, CA) ; Bernier; Eric;
(Ottawa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Huawel Technologies Co., Ltd. |
Shenzhen |
|
CN |
|
|
Family ID: |
1000005000584 |
Appl. No.: |
16/921096 |
Filed: |
July 6, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/CN2018/084324 |
Apr 25, 2018 |
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16921096 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/31 20130101; G02F
1/0121 20130101; G02F 2001/311 20130101; G02F 1/225 20130101 |
International
Class: |
G02F 1/225 20060101
G02F001/225; G02F 1/01 20060101 G02F001/01; G02F 1/31 20060101
G02F001/31 |
Claims
1. An apparatus for controlling a pair of phase shifters of an
optical device, the apparatus comprising a drive circuit including:
a first input terminal and a second input terminal; and circuitry
configured to: generate a drive signal having a magnitude based on
a level setting input signal received at the first input terminal,
wherein the circuitry generates the drive signal for driving the
pair of phase shifters; and drive one of the pair of phase shifters
using the drive signal, said one of the pair of phase shifters
being selected based on a device state control signal received at
the second input terminal.
2. The apparatus of claim 1, wherein the drive circuit comprises a
Digital-to-Analog Converter (DAC) configured to control the
magnitude of the drive signal based on a digital input.
3. The apparatus of claim 1, wherein the drive circuit comprises: a
level control circuit configured to generate an analog output
signal based on the level setting input signal, wherein magnitude
of the drive signal is based on the magnitude of the analog output
signal; and a state control circuit configured to control which one
of the pair of phase shifters is predominantly driven at a time
using the drive signal based on the device state control
signal.
4. The apparatus of claim 3, wherein the level control circuit is
operated at a first speed to vary the analog output signal, and
wherein the state control circuit is operated at a second speed,
higher than the first speed, to vary which one of the pair of phase
shifters is driven using the drive signal.
5. The apparatus of claim 1, wherein the drive circuit is a
bi-polar drive circuit configured to control a polarity of the
drive signal based on the device state control signal and provide
the drive signal via an output terminal of the bi-polar drive
circuit, and wherein the pair of phase shifters are asymmetrically
conducting, the apparatus further comprising a phase shifter
circuit comprising the pair of phase shifters connected together in
reverse-parallel and driven by the drive signal via the output
terminal of the bi-polar drive circuit.
6. The apparatus of claim 5, wherein the bi-polar drive circuit
comprises: a level control circuit configured to receive the level
setting input signal and to provide an analog output signal,
wherein magnitude of the drive signal is based on the magnitude of
the analog output signal; and a state control circuit operatively
coupled to the level control circuit and configured to control
polarity of the drive signal based on the device state control
signal.
7. The apparatus of claim 6, wherein the state control circuit
comprises: a polarity switch comprising: a first switch input
terminal electrically coupled to the level control circuit and
receiving the analog output signal therefrom; a second switch input
terminal electrically coupled to the level control circuit via a
voltage inverting circuit and receiving a signal equal in magnitude
and opposite in polarity to the analog output signal; a switch
control input terminal configured to receive the device state
control signal; and a switch output terminal operatively coupled to
the output terminal of the bi-polar drive circuit, wherein the
polarity switch is configured to operatively couple the first
switch input terminal to the output terminal of the level control
circuit when the device state control signal is in a first state,
and the polarity switch is configured to operatively couple the
second switch input terminal to the output terminal of the level
control circuit via the voltage inverting circuit when the device
state control signal is in a second state.
8. The apparatus of claim 6, wherein the state control circuit
comprises: a polarity switch comprising: a first switch input
terminal electrically coupled to the level control circuit and
receiving the analog output signal therefrom; a switch control
input terminal configured to receive the device state control
signal; a first switch output terminal operatively coupled to the
output terminal of the bi-polar drive circuit via a non-inverting
electrical amplifier to drive the optical device; and a second
switch output terminal operatively coupled to the output terminal
of the bi-polar drive circuit via an inverting electrical amplifier
to drive the optical device, wherein the polarity switch is
configured to drive the non-inverting electrical amplifier with the
analog output signal and to cause the inverting electrical
amplifier to be undriven when the device state control signal is in
a first state; and the polarity switch is configured to drive the
inverting electrical amplifier with the analog output signal and to
cause the non-inverting electrical amplifier to be undriven when
the device state control signal is in a second state.
9. The apparatus of claim 6, wherein the state control circuit
comprises: a non-inverting electrical amplifier comprising a
non-inverting input terminal electrically coupled to the level
control circuit and receiving the analog output signal therefrom;
an inverting electrical amplifier comprising an inverting input
terminal electrically coupled to the level control circuit and
receiving the analog output signal therefrom; and a polarity switch
comprising: a first switch input terminal electrically coupled to
an output terminal of the non-inverting electrical amplifier; a
second switch input terminal electrically coupled to an output
terminal of the inverting electrical amplifier; a switch control
input terminal configured to receive the device state control
signal; and a switch output terminal electrically coupled to the
output terminal of the bi-polar drive circuit to drive the optical
device, wherein the polarity switch is configured to route output
of the non-inverting electrical amplifier to the output terminal of
the bi-polar drive circuit when the device state control signal is
in a first state, and the polarity switch is configured to route
output of the inverting electrical amplifier to the output terminal
of the bi-polar drive circuit when the device state control signal
is in a second state.
10. The apparatus of claim 6, wherein the state control circuit
comprises: a first voltage controlled current source electrically
coupled to the level control circuit and configured to set a first
current level thereof based on the analog output signal, wherein
output of the first voltage controlled current source is coupled to
the output of the bi-polar drive circuit; a second voltage
controlled current source electrically coupled to the level control
circuit and configured to set a second current level thereof based
on the analog output signal, wherein the second current level is
set at twice the first current level; and a switch comprising a
switch control input terminal configured to receive the device
state control signal, a first switch terminal operatively coupled
to the output terminal of the bi-polar drive circuit and to the
first voltage controlled current source, and a second switch
terminal operatively coupled to the second voltage controlled
current source, wherein the switch is switchable between an open
position and a closed position based on the device state control
signal, and wherein: when the switch is in the open position, the
second voltage controlled current source is disconnected from the
output terminal of the bi-polar drive circuit; and when the switch
is in the closed position, the second voltage controlled current
source is connected to the output terminal of the bi-polar drive
circuit.
11. The apparatus of claim 1, wherein the drive circuit comprises a
switching circuit configured to operatively couple the drive signal
to a selectable one of the pair of phase shifters based on the
device state control signal.
12. An optical device comprising an apparatus for controlling a
pair of phase shifters of the optical device, the apparatus
comprising a drive circuit including: a first input terminal and a
second input terminal; and circuitry configured to: generate a
drive signal having a magnitude based on a level setting input
signal received at the first input terminal, wherein the circuitry
generates the drive signal specifically for driving the pair of
phase shifters; and drive one of the pair of phase shifters using
the drive signal, said one of the pair of phase shifters being
selected based on a device state control signal received at the
second input terminal.
13. A method for controlling a pair of phase shifters of an optical
device, the method comprising, by a drive circuit: receiving a
level setting input signal specific to the pair of phase shifters;
setting magnitude of a drive current based on the level setting
input signal, the magnitude of the drive current being set
specifically for the pair of phase shifters; monitoring a device
state control signal having a first state and a second state; and
causing the drive current to flow through only a first one of the
pair of phase shifters when the device state control signal is in
the first state and to flow through only a second one of the pair
of phase shifters when the device state control signal is in the
second state.
14. The method of claim 13, wherein the optical device further
comprises a second pair of phase shifters, the method further
comprising, by a second drive circuit: receiving a second level
setting input signal specific to the second pair of phase shifters;
setting magnitude of a second drive current based on the second
level setting input signal, the magnitude of the second drive
current being set specifically for the second pair of phase
shifters; monitoring a second device state control signal having
the first state and the second state; and causing the second drive
current to flow through only a first one of the second pair of
phase shifters when the second device state control signal is in
the first state and to flow through only a second one of the second
pair of phase shifters when the second device state control signal
is in the second state.
15. The method of claim 13, wherein the pair of phase shifters are
connected together in reverse-parallel to an output of the drive
circuit, and wherein causing the drive current to flow through only
the first one of the pair of phase shifters comprises causing the
drive current to flow in a first direction, and wherein causing the
drive current to flow through only the second one of the pair of
phase shifters comprises causing the drive current to flow in
opposition to the first direction.
16. The method of claim 15, wherein the drive current is directed
through a first one of the pair of phase shifters when the drive
current flows in the first direction and the drive current is
directed through a second one of the pair of phase shifters when
the drive current flows opposite to the first direction.
17. The method of claim 15, wherein the causing the drive current
to flow comprises: generating a drive voltage at the output of the
drive circuit coupled to both of the pair of phase shifters, the
drive current flowing in response to the drive voltage.
18. The method of claim 13, wherein the causing the drive current
to flow through only the first one of the pair of phase shifters
comprises: operatively coupling the drive circuit to the first one
of the pair of phase shifters using a switch, and wherein causing
the drive current to flow through only the second one of the pair
of phase shifters comprises operatively coupling the drive circuit
to the second one of the pair of phase shifters using said
switch.
19. The method of claim 13, wherein the receiving the level setting
input signal and the setting magnitude of the drive current are
performed by a level control portion of the drive circuit, and
wherein monitoring the device state control signal and causing the
drive current to flow are performed by a state control portion of
the drive circuit.
20. The method of claim 13, further comprising: operating a level
control circuit at a first speed to vary magnitude of the drive
current; and operating a state control circuit at a second speed,
higher than the first speed, to vary which of the pair of phase
shifters receives the drive current.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of PCT Application No.
PCT/CN2018/084324, filed on Apr. 25, 2018, which application is
hereby incorporated herein by reference.
TECHNICAL FIELD
[0002] The present disclosure pertains to the field of optical
devices such as optical switches and in particular to control of
multiple optical phase shifters of such a device, the optical phase
shifters for example being in a push-pull configuration.
BACKGROUND
[0003] To meet continued growth in data network traffic demands,
new data switching technologies with large switching capacities and
high interconnect bandwidths are required. Optical switching
systems represent one such technology. Silicon-on-insulator (SOI)
technologies provide a low-cost platform for high-performance
optical switching technologies for data center communication
applications. For such applications, switches that are high-speed,
broadband, low crosstalk, and have low-power consumptions are
desired.
[0004] On an SOI platform, optical devices (such as but not
necessarily limited to devices including Mach-Zehnder
interferometers (MZI)) that have carrier injection phase shifters
operating in a push-pull configuration are commonly used for
high-speed switches and other applications. While providing an
efficient optical phase shift with speeds up to potentially
hundreds of megahertz, carrier injection also produces inherent
insertion loss due to free-carrier absorption. Further, due to
fabrication imperfections and non-uniformities, there is phase
error and power imbalance between waveguide arms of the MZI,
resulting in some light leakage at the unwanted output port, which
in turn causes insertion loss, optical crosstalk, and reduced
extinction ratio.
[0005] Therefore, there is a need for an optical device such as an
optical switch, and method and apparatus for controlling same, that
obviates or mitigates one or more limitations of the prior art.
[0006] This background information is provided to reveal
information believed by the applicant to be of possible relevance
to the present disclosure. No admission is necessarily intended,
nor should be construed, that any of the preceding information
constitutes prior art against the present disclosure.
SUMMARY
[0007] In accordance with embodiments of the disclosure, there is
provided a method and apparatus for control (e.g. push-pull
control) of plural optical phase shifters in an optical device. In
accordance with embodiments of the present disclosure, there is
provided an apparatus for controlling a pair of phase shifters of
an optical device, such as but not necessarily limited to an
optical switching cell. The apparatus includes a drive circuit
having a first input terminal, a second input terminal, and
operating circuitry. The circuitry is configured to generate a
drive signal having a magnitude which is set based on a level
setting input signal received at the first input terminal. The
circuitry generates the drive signal for driving the pair of phase
shifters of the optical device, rather than for driving phase
shifters for plural optical devices. The circuitry is configured to
drive one of the pair of phase shifters using the drive signal,
where this one of the pair of phase shifters being selected based
on a device state control signal received at the second input
terminal.
[0008] The drive circuit may include a Digital-to-Analog Converter
(DAC) (e.g. one channel of a multi-channel DAC) configured to
control the magnitude of the drive signal based on a digital input.
In various embodiments, the pair of phase shifters have
substantially the same operating characteristics due to
co-manufacture and proximity to each other, and thus the same
magnitude of drive signal is appropriate for driving both phase
shifters.
[0009] In various embodiments, the drive circuit includes a level
control circuit for controlling magnitude of the drive signal, and
a separate state control circuit for controlling which one of the
pair of phase shifters is predominantly driven at a time. The level
control circuit and state control circuits can vary their outputs
at different speeds.
[0010] In various embodiments, the drive circuit is a bi-polar
drive circuit configured to control a polarity of the drive signal
based on the device state control signal and provide the drive
signal via an output terminal of the bi-polar drive circuit. In
such embodiments, the phase shifters are asymmetrically conducting
(e.g. carrier injection phase shifters), and the apparatus further
includes a phase shifter circuit comprising the pair of phase
shifters connected together in reverse-parallel and driven by the
drive signal via the output terminal of the bi-polar drive
circuit.
[0011] In various embodiments, the drive circuit includes a
switching circuit configured to operatively couple the drive signal
to a selectable one of the pair of phase shifters based on the
device state control signal.
[0012] In accordance with embodiments of the present disclosure,
there is provided an optical device comprising the above-described
apparatus. The optical device may be, for example, a Mach-Zehnder
Interferometer (MZI) device, a MZI based optical switching cell, a
Micro Ring Resonator (MRR), a MZI device comprising a MRR, a
plurality of MRRs in series or parallel, an optical switch, an
optical attenuator, a variable optical attenuator, a modulator, or
a pilot tone modulator.
[0013] In accordance with embodiments of the present disclosure,
there is provided a method for controlling a pair of phase shifters
of an optical device. The method includes receiving a level setting
input signal specific to the pair of phase shifters. The method
further includes setting magnitude of a drive current based on the
level setting input signal, the magnitude of the drive current
being set specifically for the pair of phase shifters. The method
further includes monitoring a device state control signal having a
first state and a second state. The method further includes causing
the drive current to flow through only a first one of the pair of
phase shifters when the device state control signal is in the first
state and to flow through only a second one of the pair of phase
shifters when the device state control signal is in the second
state.
[0014] In embodiments where the pair of phase shifters are
connected together in reverse-parallel to an output of the drive
circuit, causing the drive current to flow through only the first
one of the pair of phase shifters includes causing the drive
current to flow in a first direction, and causing the drive current
to flow through only the second one of the pair of phase shifters
comprises causing the drive current to flow in opposition to the
first direction.
[0015] In some embodiments, causing the drive current to flow
through only the first one of the pair of phase shifters includes
operatively coupling the drive circuit to the first one of the pair
of phase shifters using a switch, and causing the drive current to
flow through only the second one of the pair of phase shifters
includes operatively coupling the drive circuit to the second one
of the pair of phase shifters using said switch.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Further features and advantages of the present disclosure
will become apparent from the following detailed description, taken
in combination with the appended drawings, in which:
[0017] FIG. 1A illustrates an example push-pull configuration of a
Mach-Zehnder Interferometer (MZI) based Carrier Injection Optical
Switch (CIOS) cell and an associated drive circuit according to an
embodiment of the present disclosure.
[0018] FIGS. 1B-1 and 1B-2 illustrate optical operation of the MZI
based CIOS cell of FIG. 1A in "Cross" and "Bar" states, according
to an embodiment of the present disclosure.
[0019] FIG. 1C illustrates an implementation of a pair of carrier
injection phase shifters operatively coupled to a pair of
waveguides and connected in a reverse-parallel configuration,
according to an embodiment of the present disclosure.
[0020] FIG. 2 illustrates, in a block diagram, an example
configuration of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to an
embodiment of the present disclosure.
[0021] FIG. 3A illustrates, in a component view, an example
implementation of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to an
embodiment of the present disclosure.
[0022] FIG. 3B illustrates, in a component view, an example
implementation of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to
another embodiment of the present disclosure.
[0023] FIG. 3C illustrates, in a component view, an example
implementation of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to
another embodiment of the present disclosure.
[0024] FIG. 3D illustrates, in a component view, an example
implementation of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to
another embodiment of the present disclosure.
[0025] FIG. 3E illustrates, in a component view, an example of an
alternative implementation of a unipolar drive circuit for
controlling an MZI-based push-pull CIOS cell in electrical domain
according to another embodiment of the present disclosure.
[0026] FIG. 3F illustrates, in a component view, an example of an
alternative implementation of a unipolar drive circuit for
controlling an MZI-based push-pull CIOS cell in electrical domain
according to another embodiment of the present disclosure.
[0027] FIG. 4 illustrates, in a flow chat, a method for controlling
an MZI-based push-pull CIOS cell according to an embodiment of the
present disclosure.
[0028] FIG. 5 illustrates, in a component view, an example
configuration for driving a plurality of MZI-based push-pull CIOS
cells in electrical domain according to an embodiment of the
present disclosure.
[0029] FIGS. 6A and 6B illustrate a Micro-ring Resonator device
comprising a pair of phase shifters, which can be driven according
to embodiments of the present disclosure.
[0030] It will be noted that throughout the appended drawings, like
features are identified by like reference numerals.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0031] Embodiments of the present disclosure provide a method of
and an apparatus for controlling optical devices that have plural
phase shifters, for example operating in a push-pull configuration.
These optical devices may include but are not limited to a
Mach-Zehnder Interferometer (MZI) device, an MZI based optical
switching cell, a Micro Ring Resonator (MRR), an MZI device
comprising an MRR, a plurality of MRRs in series or parallel, an
optical switch, an optical attenuator, a variable optical
attenuator, a modulator, or a pilot tone modulator, where a
push-pull driver is required. For clarity, the example of
application to an optical switch is considered in detail. In
various embodiments, only one of the plural phase shifters is
activated (e.g. forward biased) at a time. A push-pull
configuration can refer to a configuration in which plural phase
shifters are integrated into different parts of the optical device
(e.g. different parallel arms of an MZI device) and operated in a
complementary manner.
[0032] Embodiments of the present disclosure provide for an
apparatus to independently adjust drive signal magnitude for each
separate optical device (e.g. switching cell), while also allowing
for switching between push and pull phase shifter states using a
switch which is separate from the drive signal magnitude adjustment
circuit. Because drive signal magnitude is performed on a
per-device basis, crosstalk can be potentially reduced and contrast
ratio can be potentially increased. Because the drive signal
magnitude adjustment circuit is used for a pair of phase shifters
rather than for a single phase shifter, the circuitry is
simplified.
[0033] In some embodiments, a bi-polar drive circuit is used to
control a polarity of a drive signal based on a device state
control signal. The drive signal is then provided via an output
terminal of the bi-polar drive circuit. Furthermore, the phase
shifters are provided as asymmetrically conducting phase shifters.
Such phase shifters allow (and are responsive to) current flow in
substantially one direction only, and are represented in the
electrical domain using a diode symbol. A primary example of an
asymmetrically conducting phase shifter is a carrier injection
phase shifter. A phase shifter configured as a Metal Oxide
Semiconductor (MOS) device may also be an asymmetrically conducting
phase shifter. Alternatively, a phase shifter connected in series
with a diode or other asymmetrically conducting electrical
component may be used as an asymmetrically conducting phase
shifter. In such embodiments, the apparatus includes a phase
shifter circuit having the pair of asymmetrical phase shifters
connected together in reverse-parallel and driven by the drive
signal via the output terminal of the bi-polar drive circuit. In
other embodiments, a uni-polar drive circuit is used to selectably
apply the drive signal to primarily one phase shifter at a
time.
[0034] Reverse-parallel connection, also referred to as connection
in parallel in opposite polarities, is a connection in which the
anode of a first phase shifter and the cathode of a second phase
shifter are coupled together to one node of the phase shifter
circuit, and the cathode of the first phase shifter and the anode
of the second phase shifter are coupled together to another node of
the phase shifter circuit. The term "anode" and "cathode" are
intended to capture the characteristics of two terminals of an
asymmetrically conducing device such as a diode or carrier
injection phase shifter, in which current is primarily allowed to
flow, by convention and in forward biasing, from anode to cathode.
Other types of phase shifters that may be employed include Carrier
depletion junction phase shifters, Stark Effect phase shifters, and
Franz Keldysh phase shifters, for example.
[0035] Embodiments of the present disclosure will be explained in
more detail with respect to the accompanying drawings.
[0036] FIG. 1A illustrates an example push-pull configuration of a
Mach-Zehnder Interferometer (MZI)-based Carrier Injection Optical
Switch (CIOS) cell 120 and an associated drive circuit according to
at least one embodiment of the present disclosure. The cell 120 may
be one of a potentially large number of similar cells which are
interconnected in a switching fabric, for example, with each cell
associated with its own driving circuit.
[0037] As illustrated in FIG. 1A, the MZI-based CIOS cell 120 may
include a first optical coupler 105 that receives and couples
optical signals from a pair of input ports 101 and 102, and a
second optical coupler 125 that couples signals provided at its
inputs and provides optical signals at a pair of output ports 127
and 128. The couplers 105 and 125 may be 50% directional couplers,
multimode interferometers (MMIs), or the like. Additionally, the
MZI-based CIOS cell 120 may include a first optical waveguide arm
103 and a second optical waveguide arm 104 coupled in parallel
between the first coupler 105 and the second coupler 125. Design
and operation of the MZI-based CIOS cell 120, apart from design and
operation of the associated drive circuit, will be readily
understood by a worker skilled in the art.
[0038] In some embodiments, as illustrated in FIG. 1A, an apparatus
100 for controlling the MZI-based CIOS cell 120 operating in a
push-pull configuration may include a bi-polar drive circuit 110
and a phase shifter circuit 136.
[0039] The bi-polar drive circuit 110 may include a first input
terminal 133 configured to receive a level setting input signal, a
second input terminal 134 configured to receive a device state
control signal having a first state and a second state, and an
output terminal 137 electrically coupled to the phase shifter
circuit 136 to provide a drive signal. This drive signal may be a
current or a voltage signal that causes a drive current to flow
through the phase shifter circuit 136. The bi-polar drive circuit
110 may be configured to control a magnitude of the drive signal
based on the level setting input signal and to control a polarity
of the drive signal based on the device state control signal.
[0040] In the example embodiment as illustrate in FIG. 1A, the
phase shifter circuit 136 may include a first carrier injection
phase shifter D1 integrated with the first waveguide arm 103 and a
second carrier injection phase shifter D2 integrated with the
second waveguide arm 104 of the MZI-based CIOS cell 120. The
carrier injection phase shifters are represented as diodes in the
electrical domain (i.e. in the equivalent electrical circuit) for
purposes of clarity, and to reflect their typical behavior in the
electrical domain as asymmetrically conducting devices. However it
should be understood that these phase shifters are devices which
impart an optical phase shift to the respective optical arms of the
MZI-based CIOS cell 120 based on carrier injection effects. The
first carrier injection phase shifter D1 and the second carrier
injection phase shifter D2 may be connected in parallel in opposite
polarities with a first common node 135 electrically coupled to the
output terminal 137 of the bi-polar drive circuit 110 and with a
second common node electrically coupled to a high-speed common mode
reference node 132. The pair of carrier injection phase shifters D1
and D2 may be, for example, p-i-n or p-n type phase shifters. The
pair of phase shifters D1 and D2 may have substantially the same
operating characteristics due to co-manufacture and proximity to
each other. The pair of carrier injection phase shifters D1 and D2
may be operated in a push-pull manner, such that only one of the
phase shifters is forward-biased at a time. Which one of the phase
shifters is forward-biased is determined based on whether the
device state control signal is in the first state or the second
state. Typical switching time of the carrier injection phase
shifters is on the order of a few nanoseconds, which may require
very fast switching of current direction into phase shifter
junction diodes.
[0041] Note that, by way of example, in this instant application,
the first state of the device state control signal may refer to a
"Cross" or "OFF" state, and the second state of the device state
control signal may refer to a "Bar" or "ON" state.
[0042] Additionally, in some embodiments, a bias element 129 may be
operatively coupled to (e.g. integrated with) one of the waveguide
arms 103 and 104 (in this example embodiment, the first waveguide
arm 103). The bias element 129 imparts an additional phase shift to
optical signals traversing the first waveguide arm 103, for example
due to a thermo-optic (TO) effect or another effect. This
additional phase shift can be configured, for example based on a
calibration operation, to impart a desired amount of phase shift
for supporting the desired operation of the MZI-based CIOS
switching cell 120. The bias element 129 may be configured to
compensate for inequality of length of the waveguide arms 103 and
104, for example. An optical signal propagating through the first
waveguide arm 103 may be delayed by the bias element 129 to match
the delay in the second waveguide arm 104. Thus, the bias element
129 may be a thermal-optic (TO) phase shifter. By adjusting a tuner
input 123 of the bias element 129, the desired phase shift imparted
by the bias element can be controlled. For example, such adjustment
may be performed to balance phase shift in the pair of waveguide
arms 103 and 104. It is noted that the bias element 129 can
alternatively be located between the first coupler 105 and the
first carrier injection phase shifter D1.
[0043] As illustrated by the example embodiment in FIG. 1A, an
optical signal entering the first input port 101 is split by the
input coupler 105 into two signals of substantially equal magnitude
propagating in the first waveguide arm 103 and second waveguide arm
104. With the TO phase shifter 129 having a phase shift of -.pi./2,
when the drive current is applied to the first carrier injection
phase shifter D1, the optical signal portion propagating in the
first waveguide arm 103 may be delayed in phase by .pi./2, which
results in the optical signal exiting the second output port 128.
This is known as an "OFF" or "Cross" state; when the drive current
is applied to the second carrier injection phase shifter D2, the
optical signal portion propagating in the second waveguide arm 104
will be delayed in phase by .pi./2, which results in the optical
signal exiting the first output port 127. This is known as the "ON"
or "Bar" state. When the drive signal is a drive current, the drive
current may be obtained by applying the current directly to (or
drawing the current directly from) the common node 135 and the
polarity of the drive current determines which one of the carrier
injection phase shifters D1 and D2 is forward biased and therefore
applying a corresponding amount of optical phase shift.
Alternatively, when the drive signal is a voltage signal, the drive
current may be obtained by applying a drive voltage to the common
node 135. For example, if a voltage level at the common node 135 is
higher than a voltage level at the high speed common mode reference
node 132, the drive current may flow from the common node 135,
through the carrier injection phase shifter D1 (which is forward
biased), and toward the high speed common mode reference node 132.
At the same time, the phase shifter D2 is reverse biased and thus
conducts little to no current. The reverse-biased phase shifter may
impart a limited, but possibly non-zero phase shift. If the voltage
level at the common node 135 is lower than the voltage level at the
high speed common mode reference node 132, the drive current may
flow from the high speed common mode reference node 132, through
the carrier injection phase shifter D2 (which is now forward
biased), and toward the common node 135. At the same time, the
phase shifter D1 is now reverse biased and thus conducts little to
no current.
[0044] FIGS. 1B-1 and 1B-2 illustrate an example optical operation
of an MZI-based CIOS cell, for purposes of clarity. The bias
element TO 129 is configured to apply a phase shift of -.pi./2
radians to optical signals propagating through the waveguide arm
103. The couplers 105 and 125 each have upper inputs and outputs
and lower inputs and outputs, and are configured such that part
(e.g. 50%) of a signal applied to the upper (or lower input) is
provided without phase shift at the upper (or lower) output, and
the remainder of the signal applied to the upper (or lower input)
is provided with a phase shift of -.pi./2 radians at the lower (or
upper) output.
[0045] In FIG. 1B-1, the phase shifter D1, which may be a PIN type
carrier injection phase shifter, is forward-biased and driven, for
example using the bi-polar drive circuit 110, by a configured
amount of current (or voltage, or both), such that it applies a
phase shift of .pi./2 radians to optical signals propagating
through the waveguide arm 103. At the same time, the phase shifter
D2, which may also be a PIN type carrier injection phase shifter,
is reverse-biased and substantially undriven, for example using the
bi-polar drive circuit 110, such that it applies a phase shift of 0
radians to optical signals propagating through the waveguide arm
104. Taken together with operation of the bias element TO 129 and
the couplers 105 and 125, this results in a signal applied to the
upper (or lower) input of the coupler 105 being routed to the lower
(or upper) output of the coupler 125, resulting in the "Cross"
state of the optical switch.
[0046] In FIG. 1B-2, the phase shifter D2 is forward-biased and
driven, for example using the bi-polar drive circuit 110, by a
configured amount of current (or voltage, or both), such that it
applies a phase shift of .pi./2 radians to optical signals
propagating through the waveguide arm 104. At the same time, the
phase shifter D1 is reverse-biased and substantially undriven, for
example using the bi-polar drive circuit 110, such that it applies
a phase shift of 0 radians to optical signals propagating through
the waveguide arm 103. Taken together with operation of the bias
element TO 129 and the couplers 105 and 125, this results in a
signal applied to the upper (or lower) input of the coupler 105
being routed to the upper (or lower) output of the coupler 125,
resulting in the "Bar" state of the optical switch.
[0047] Whereas the embodiments have been described with respect to
the Cross and Bar states with an input signal illuminating a first
optical input, the device may equally be configured and operated by
illuminating a second optical input so that to respective states
are Bar and Cross states.
[0048] FIG. 1C schematically illustrates a pair of carrier
injection phase shifters D1 and D2 integrated with a pair of
waveguide arms 103, 104 and connected in reverse-parallel,
according to an embodiment of the present disclosure. The
equivalent electrical circuit is also shown. Each phase shifter
includes a P-doped region and an N-doped region disposed on
opposing sides of the waveguide to which it is coupled. The P-doped
region of phase shifter D1 is electrically connected to the N-doped
region of phase shifter D2, and the N-doped region of phase shifter
D1 is electrically connected to the P-doped region of phase shifter
D2.
[0049] Alternatively, as shown in FIGS. 3E and 3F, the phase
shifters can be electrically connected in parallel to two separate
outputs of a single pole double throw switch, where the input to
the switch is connected to a source of drive voltage or current,
and operation of the switch is used to select which one of the two
phase shifters is being driven. Note that in FIGS. 3E and 3F, the
phase shifters are connected in parallel with common polarity, and
are not connected in reverse parallel.
[0050] Although an MZI-based Carrier Injection Optical Switch
(CIOS) cell is illustrated in FIG. 1A, it will be readily
understood that other types of optical devices may be used, such as
a Micro Ring Resonator (MRR), an MZI device comprising an MRR, a
plurality of MRRs in series or parallel, an optical switch, an
optical attenuator, a variable optical attenuator, a modulator, or
a pilot tone modulator, provided that device requires a push-pull
drive. In this case, the same circuitry can be used, and the phase
shifters D1 and D2 may remain electrically coupled in reverse
parallel as shown. However some or all of the optical components of
the MZI device, such as the couplers 105 and 125, the bias element
129 and the optical waveguide arms 103 and 104, may be varied. The
embodiments illustrated in FIGS. 2 and 3A to 3F do not explicitly
show the device optical components, and the phase shifters D1 and
D2 illustrated therein can be integrated with a CIOS switch cell as
in FIG. 1, or to another optical device.
[0051] In addition, although a bi-polar drive circuit is
illustrated in FIG. 1A to drive a pair of carrier injection phase
shifters connected in parallel in opposite polarities with
push-pull configuration, a unipolar drive circuit may alternatively
be used to drive a pair of phase shifters connected in parallel
with common polarity where the phase shifters are driven in a
push-pull manner. A description of the unipolar drive circuit will
be provided with respect to FIGS. 3E and 3F.
[0052] More generally, embodiments of the present disclosure
provide for apparatus for controlling a pair of phase shifters of
an optical device, such as but not limited to an MZI-based optical
switch. The pair of phase shifters are operated in push-pull
configuration. This may involve causing only one of the phase
shifters to be forward biased at a time, in the case of
asymmetrically conducting phase shifters. The apparatus includes a
drive circuit having a first input terminal and a second input
terminal, and operating circuitry. The circuitry is configured to
generate a drive signal having a magnitude which is set based on a
level setting input signal received at the first input terminal.
The circuitry is further configured to drive one of the pair of
phase shifters using the drive signal. The driven one of the pair
of phase shifters is selected based on a device state control
signal received at the second input terminal.
[0053] While the device has principally been illustrated by means
of a 2.times.2 optical switch having 2 optical inputs and 2 optical
outputs, it should be understood that the device may also be
configured as a 1.times.2 switch having 1 input and 2 outputs, a
2.times.1 switch having 2 inputs and 1 output, or a 1.times.1
device having 1 input and 1 output. In cases where there is only 1
output port, light is coupled out of the output port or is
scattered away from the device, with the fraction of light that is
coupled out of the output port being controlled by the phase
shifters. Any device configuration may be used as a switch,
modulator or attenuator depending on the desired system
application.
[0054] In various embodiments, the drive circuit includes an
electrical switch. This switch may be used, for example, to
selectably couple output of the drive circuit to one of the pair of
phase shifters at a time. Alternatively, the switch may be used to
switch polarity of the drive signal, by reversing the voltage
polarity relative to a common reference voltage, or by reversing a
direction of drive current.
[0055] In various embodiments, only one of the phase shifters is
predominantly driven at a time. This may be achieved for example by
forward-biasing one of the phase shifters with a predetermined
voltage or causing a predetermined amount of current to flow
through the phase shifter in accordance with such forward-biasing.
The other phase shifter may then be reverse-biased. In this case, a
nominal but typically limited amount of current may flow through
the other, reverse-biased phase shifter, which can thus be
considered to be driven in a limited manner. However, for purposes
of clarity, it is considered herein that only the forward-biased
phase shifter is being primarily driven.
[0056] In some embodiments, the circuitry generates the drive
signal specifically for driving the pair of phase shifters. That
is, the drive signal is used for driving either one of the pair of
phase shifters, but the drive signal is not used for driving any
other phase shifters that may be part of or other optical devices
(e.g. which may be operatively coupled to or integrated with the
optical device and/or disposed on the same optical chip or
substrate). As such, each drive signal indicative of a magnitude is
associated with a single device state control signal for setting
the state of an optical device (e.g. an ON or OFF state, or one of
a plurality of discrete states). Thus, the drive signal and the
device state control signal are applied only to the same optical
device. Because the drive signal is specific to one device, the
drive signal magnitude can be tuned specifically based on
characteristics of the two phase shifters of that device. The same
tuning may be applicable to each of the two phase shifters, but at
different times, due to the fact that both phase shifters have
similar characteristics. These similar characteristics may be due
to co-manufacture and co-location (i.e. close proximity) of the two
phase shifters in the manufactured optical device. In some
embodiments, the same drive signal is used to drive multiple
(possibly more than two) phase shifters having similar
characteristics due to co-manufacture and co-location.
[0057] FIG. 2 illustrates, in a block view, an example
configuration of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to at
least one embodiment of the present disclosure. As illustrated in
FIG. 2, the bi-polar drive circuit 110 may include a level control
circuit 210 configured to receive the level setting input signal
and to provide an analog level setting output signal 233. The
bi-polar drive circuit 110 may also include a state control circuit
220 configured to control a polarity of the drive signal based on
the device state control signal received at the second input
terminal 134. The level control circuit 210 may be operated at a
first speed to vary the analog level setting output signal, and the
state control circuit 220 may be operated at a second speed, higher
than the first speed, to vary the polarity of the drive signal. For
example, this may allow the level control circuit 210 to employ
more complex and hence slower circuitry such as a DAC, while the
state control circuit 220 employs faster circuitry such as
switches. Additionally, the state control circuit 220 may be
further configured to convert a voltage to a current or a voltage
and switch the polarity of the current or voltage based on the
device state control signal.
[0058] Thus, as illustrated in FIG. 2 and described herein above,
the bi-polar drive circuit 110 may be configured to control the
magnitude of the drive signal based on the analog level setting
output signal, and to cause a drive current to flow. The drive
current is caused to flow in a first direction and with said
magnitude, primarily through one of the carrier injection phase
shifters when the device state control signal is in the first
state. The drive current is caused to flow opposite to the first
direction and with said magnitude, primarily through another one of
the carrier injection phase shifters when the device state control
signal is in the second state.
[0059] More generally, the drive circuit, which may or may not be a
bi-polar drive circuit, may include two substantially separate but
interconnected circuits, namely a level control circuit and a state
control circuit. The level control circuit is configured to
generate an analog output signal based on a (possibly digital)
level setting input signal. Then, magnitude of the drive signal is
based on the magnitude of the analog output signal. The level
control circuit may include a DAC or a single channel of a
multi-channel DAC. The state control circuit is configured to
control, based on the device state control signal, which one of the
pair of phase shifters is predominantly driven at a time using the
drive signal of the set magnitude. The state control circuit may
include at least one switch and be responsive to an input signal
having discrete signal states causing the at least one switch to
enter a corresponding switching state. In various embodiments, the
level control circuit is operated at a first speed to vary the
analog output signal, and the state control circuit is operated at
a second speed, higher than the first speed, to vary which one of
the pair of phase shifters is driven using the drive signal. This
allows the state of the optical device to be altered at the speed
of the state control circuit. At the same time, the level control
circuit can be operated, on a slower time scale, to tune the
control circuit so that the optimal (or substantially optimal)
drive signal magnitude is applied to the optical device. This drive
signal magnitude can be previously determined based on prior
calibration operations. Once received, the same drive signal
magnitude can be held constant until updated.
[0060] FIG. 3A illustrates, in a component view, an example
implementation of a bi-polar drive circuit 110A for controlling an
MZI-based push-pull CIOS cell in electrical domain according to an
embodiment of the present disclosure. As illustrated in FIG. 3A,
the bi-polar drive circuit 110A may include a level control circuit
210 comprising a Digital-to-Analog Converter (DAC) 310 configured
to produce and hold an analog output signal as the analog level
setting output signal 233. The analog output signal accordingly has
a level that corresponds to a digital input signal provided to the
DAC. The digital input signal is provided as the level setting
input signal received at input terminal 133. In some embodiments,
the DAC may be provided as one channel of a multi-channel DAC and
is accordingly referred to as one DAC channel 310 in this instant
application.
[0061] The one DAC channel 310 may include an input terminal
electrically coupled to the first input terminal 133 of the
bi-polar drive circuit 110A to receive the level setting input
signal. The level setting input signal may be a digital signal
being provided based on calibration information, for example as
stored in a calibration look-up table. The calibration information
may be determined according to a prior calibration procedure. The
calibration information may indicate an optimal, desired or
adequately optimal magnitude for the drive signal being output by
the drive circuit. The one DAC channel 310 may further include an
output terminal configured to provide an analog level setting
output signal 233 having a level that corresponds to the level
setting input signal, according to the digital-to-analog conversion
operation of the DAC channel 310.
[0062] As illustrated in FIG. 3A, the bi-polar drive circuit 110A
may further include a state control circuit 220A. The state control
circuit 220A may include a polarity switch 330A (single pole double
throw) having a first input terminal electrically coupled to the
output terminal of the one DAC channel 310 to receive the analog
level setting output signal 233. The state control circuit 220A may
include a second input terminal electrically coupled to the output
terminal of the one DAC channel 310 via a voltage inverter 320 to
receive an inversing analog level setting output signal having
substantially the same magnitude as that of the analog level
setting output signal 233 but the opposite voltage polarity,
relative to the voltage at the high speed common mode reference
node 132. The state control circuit 220A may include a third input
terminal electrically coupled to the second input terminal of the
state control circuit 220A to receive the device state control
signal via terminal 134. The circuit 220A may include an output
terminal electrically coupled to the output terminal of the
bi-polar drive circuit 110A via a non-inverting electrical
amplifier 340 to drive the pair of carrier injection phase shifters
D1 and D2 that are integrated with the MZI-based CIOS cell.
Accordingly, a positive voltage signal (relative to the voltage at
reference node 132) may be derived at the output terminal of the
bi-polar drive circuit when the device state control signal is in
the first state; and a negative voltage signal (relative to the
voltage at reference node 132) may be derived at the output
terminal of the bi-polar drive circuit when the device state
control signal is in the second state. The voltage signal derived
at the output terminal of the bi-polar drive circuit 110A may drive
the pair of carrier injection phase shifters D1 and D2 into ON or
OFF states, which in turn may cause the CIOS cell to operate in
"Cross" or "Bar" states, respectively.
[0063] When the positive voltage signal is output by the bi-polar
drive circuit, phase shifter D1 is forward biased and phase shifter
D2 is reverse biased. This drives the phase shifter D1 in the ON
state and places the phase shifter D2 in the OFF state. Likewise,
when the negative voltage signal is output by the bi-polar drive
circuit, phase shifter D2 is forward biased and phase shifter D1 is
reverse biased. This drives the phase shifter D2 in the ON state
and places the phase shifter D1 in the OFF state. When in the ON
state, the phase shifter D1 or D2 imparts a phase shift which is
configurable based on drive signal magnitude, which can be set to a
desired level for example via calibration.
[0064] FIG. 3B illustrates, in a component view, an example
implementation of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to
another embodiment of the present disclosure. As illustrated in
FIG. 3B, the bi-polar drive circuit 110B may include a level
control circuit 210 comprising one DAC channel 310 operating the
same way as described herein above with respect to FIG. 3A. The
bi-polar drive circuit 110B may further include a state control
circuit 220B which may be analogous to the state control circuit
220A of FIG. 3A. The state control circuit 220B may include a
polarity switch 330B (single pole double throw) comprising a first
input terminal electrically coupled to the output terminal of the
one DAC channel 310 to receive the analog level setting output
signal 233; a second input terminal electrically coupled to the
second input terminal of the state control circuit 220B to receive
the device state control signal; a first output terminal
electrically coupled to the output terminal of the bi-polar drive
circuit 110B via a non-inverting electrical amplifier 340; and a
second output terminal electrically coupled to the output terminal
of the bi-polar drive circuit 110B via an inverting electrical
amplifier 350 having substantially the same voltage gain magnitude
as that of the non-inverting electrical amplifier 340. In some
embodiments, the non-inverting electrical amplifier 340 and the
inverting electrical amplifier 350 may be implemented using
non-inverting and inverting unit-gain amplifiers, respectively.
Alternatively, the two electrical amplifiers 340, 350 may have the
same gain but not necessarily unity. Ground reference for the two
amplifiers may be equal to the voltage at the reference node 132.
Thus, a positive voltage signal (relative to voltage at reference
node 132) may be derived at the output terminal of the bi-polar
drive circuit 110B when the device state control signal is in the
first state; and a negative voltage signal (relative to voltage at
reference node 132), having the same magnitude as the positive
voltage signal but opposite sign, may be derived at the output
terminal of the bi-polar drive circuit 110B when the device state
control signal is in the second state. The voltage signal derived
at the output terminal of the bi-polar drive circuit 110B may cause
the pair of carrier injection phase shifters D1 and D2 to be driven
into appropriate ON or OFF states, which in turn may cause the CIOS
cell to operate in "Cross" or "Bar" states, respectively. Such
operation in terms of forward and reverse bias of the phase
shifters D1 and D2 is as already described above.
[0065] FIG. 3C illustrates, in a component view, an example
implementation of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to
another embodiment of the present disclosure. As illustrated in
FIG. 3C, the bi-polar drive circuit 110C may include a level
control circuit 210 comprising one DAC channel 310 operating the
same way as described herein above with respect to FIG. 3A and FIG.
3B. The bi-polar drive circuit 110C may further include a state
control circuit 220C which may be analogous to the state control
circuits 220A and 220B of FIG. 3A and FIG. 3B, respectively. The
state control circuit 220C may include a non-inverting electrical
amplifier 340 comprising a non-inverting input terminal
electrically coupled to the output terminal of the one DAC channel
310 and an output terminal. The circuit 220C may further include an
inverting electrical amplifier 350 comprising an inverting input
terminal electrically coupled to the output terminal of the one DAC
channel 310 and an output terminal. The circuit 220C may further
include a polarity switch 330A comprising a first input terminal
electrically coupled to the output terminal of the non-inverting
electrical amplifier 340; a second input terminal electrically
coupled to the output terminal of the inverting electrical
amplifier 350; a third input (control input) terminal electrically
coupled to the second input terminal of the state control circuit
220C to receive the device state control signal; and an output
terminal electrically coupled to the output terminal of the
bi-polar drive circuit 110C. In some embodiments, the non-inverting
unity gain amplifier 340 may be omitted, such one input terminal of
the switch 330A is directly coupled to the output terminal of the
one DAC channel 310. The output terminal of the bi-polar drive
circuit is thus operated to drive the pair of carrier injection
phase shifters D1 and D2, which are in turn coupled to the
MZI-based CIOS cell. More specifically, a positive voltage signal
is provided, via the non-inverting amplifier 340, at the output
terminal of the bi-polar drive circuit 110C when the device state
control signal is in the first state. A negative voltage signal is
provided, via the inverting amplifier 350, at the output terminal
of the bi-polar drive circuit 110C when the device state control
signal is in the second state. The voltage signal derived at the
output terminal of the bi-polar drive circuit 110C may drive the
pair of carrier injection phase shifters D1 and D2 into
corresponding ON or OFF states, due to forward and reverse biasing
of same. This in turn may cause the CIOS cell to operate in "Cross"
or "Bar" states, respectively.
[0066] FIG. 3D illustrates, in a component view, an example
implementation of a bi-polar drive circuit for controlling an
MZI-based push-pull CIOS cell in electrical domain according to
another embodiment of the present disclosure. As illustrated in
FIG. 3D, the bi-polar drive circuit 110D may include a level
control circuit 210 comprising one DAC channel 310 operating the
same way as described herein above with respect to FIG. 3A, FIG.
3B, and FIG. 3C. The bi-polar drive circuit 110D may further
include a state control circuit 220D which may be analogous to the
state control circuits 220A, 220B, 220C of FIG. 3A, FIG. 3B and
FIG. 3C, respectively. The state control circuit 220D may include a
first voltage controlled current source Ip electrically coupled to
the output terminal of the one DAC channel 310 to receive the
analog level setting output signal as a current source control
voltage. The state control circuit 220D may include a second
voltage controlled current source In having a current magnitude
that is double of that of the first voltage controlled current
source Ip. The second voltage controlled current source In is
electrically coupled to the one DAC channel to receive the analog
level setting signal as the current source control voltage. The
state control circuit 220D may further include an input (control)
terminal electrically coupled to the second input terminal of the
state control circuit 220D to receive the device state control
signal. An output terminal is electrically coupled to the first
voltage controlled current source Ip; and a polarity switch 330D
(single pole single throw) that connects the second voltage
controlled current source In to the first voltage controlled
current source Ip when the device state control signal is in the
second state (and the switch 330D is closed), and disconnects the
second voltage controlled current source In from the first voltage
controlled current source Ip when the device state control signal
is in the first state (and the switch 330D is open). When the first
and the second voltage controlled current sources are disconnected,
the current from the first voltage controlled current source (i.e.
Idrive=Ip) flows through the first carrier injection phase shifter
D1 towards the high speed common mode reference node 132. The first
carrier injection phase shifter D1 is turned on and imparts a phase
shift corresponding to the provided amount of current, and the
second carrier injection phase shifter D2 is substantially turned
off. The CIOS cell 120 is thereby placed in "Cross" state (when the
amount of phase shift and current are tuned appropriately). When
the first and the second voltage controlled current sources are
connected due to closing of the switch 330D, a drive current of
Idrive=In-Ip flows from the high speed common mode reference node
132 through the second carrier injection phase shifter D2 towards
the common node 135. The first carrier injection phase shifter D1
is substantially turned off and the second carrier injection phase
shifter D2 is turned on and imparts a phase shift corresponding to
the provided amount of current. The CIOS cell 120 is in "Bar" state
(when the amount of phase shift and current are tuned
appropriately).
[0067] Although various implementation examples of using one
bi-polar drive circuit to drive a pair of carrier injection phase
shifters connected in parallel in opposite polarities with
push-pull configuration have been discussed herein above, a
unipolar drive circuit may be derived from the bi-polar drive
circuit after a minor modification to drive a pair of carrier
injection phase shifters connected in parallel in a uni-polar
push-pull configuration. FIG. 3E illustrates, in a component view,
an example of an alternative implementation of a unipolar drive
circuit for controlling an MZI-based push-pull CIOS cell in
electrical domain according to another embodiment of the present
disclosure.
[0068] As illustrated in FIG. 3E, the uni-polar drive circuit 110E
may include a level control circuit 210 comprising one DAC channel
310 operating the same way as described herein above with respect
to FIGS. 3A-3D. The uni-polar drive circuit 110E may further
include a state control circuit 220E. The state control circuit
220E may include a polarity switch 330B comprising a first input
terminal 305E electrically coupled to the output terminal of the
one DAC channel 310 via a voltage controlled current source 302E; a
second input (control) terminal 307E electrically coupled to the
second input terminal of the state control circuit 220E to receive
the device state control signal; and a first and a second output
terminals 310E, 312E electrically coupled to a pair of carrier
injection phase shifters D1 and D2, respectively. The current from
the voltage controlled current source flows through the first
carrier injection phase shifter D1 when the device state control
signal is in the first state; and the current from the voltage
controlled current source flows through the second carrier
injection phase shifter D2 when the device state control signal is
in the second state. The current flow may drive the pair of carrier
injection phase shifters D1 and D2 into ON or OFF states, which in
turn may cause the CIOS cell to operate in "Cross" or "Bar" states,
respectively. This drive circuit therefore illustrates an example
of a switching circuit configured to operatively couple the drive
signal to a selectable one of the pair of phase shifters based on
the device state control signal. However, it should be appreciated
that the illustrated bi-polar drive circuits also utilize such a
switching circuit, which is configured to operatively coupled the
drive signal to a selectable one of the pair of phase shifters
based on the device state control circuit. In the case of the
bi-polar drive circuit, the selectable operative coupling is
achieved by operating the switch so that current direction or
voltage polarity is switchably changed.
[0069] FIG. 3F illustrates the same circuitry as FIG. 3E, except
that the current source 302E has been omitted and an optional
electrical amplifier 310F has been added in its place. The
amplifier can be omitted if additional gain is not necessary and
the output of the DAC 310 is sufficiently strong to drive the phase
shifters. Operation of the circuit of FIG. 3F is the same as in
FIG. 3E, except that the current source is omitted. A voltage is
switchably applied to the phase shifters D1 and D2.
[0070] In various embodiments, the state control circuit 220 may
include but not limited to a switch that is implemented in the form
of a multiplexer or demultiplexer. As described elsewhere herein,
this switch is either used to implement polarity switching for the
drive current or drive voltage output by the drive circuit, or for
switchably coupling the drive circuit to a selected one of a
plurality of phase shifters. For example, the polarity switch 330A
of FIG. 3A or 3C may be a 2:1 multiplexer. The polarity switch 330B
of FIG. 3B, 3E or 3F may be a 1:2 demultiplexer. Alternatively, the
switches may be multiplexers or demultiplexers with different
numbers of inputs and outputs. In some embodiments, the same drive
signal is used to selectably drive three or more co-manufactured
and co-located phase shifters of an optical device. In this case,
an N-way switch, such as an N:1 multiplexer or 1:N demultiplexer,
can be used to controllably route the drive signal so that a
selected one of the plurality of three or more phase shifters is
operated. An N-state switching cell control signal can be provided
to the N-way switch.
[0071] FIG. 4 illustrates, in a flow chart, a method 400 for
controlling an optical device having multiple phase shifters,
according to at least one embodiment of the present disclosure. The
phase shifters may be operated in a push-pull configuration, and
the device may for example be an MZI-based push-pull CIOS cell. A
pair of phase shifters of the optical device are controlled. The
method 400 may include receiving 410, by a drive circuit, a level
setting input signal and setting 420 a magnitude of a drive current
based on the level setting input signal. The setting a magnitude
420 of a drive current may include, by a level control portion of
the drive circuit, receiving the level setting input signal and
providing an analog level setting output signal based on the level
setting input signal. The magnitude of the drive current may be set
based on the analog level setting output signal. The drive circuit
may be a bi-polar drive circuit or a unipolar drive circuit. The
drive current magnitude may be set specifically for the pair of
phase shifters. The level setting input signal may be associated
with a single device state control signal such as mentioned below.
This results in the level setting input signal being specific to a
single optical device, or at least specific to a set of
co-manufactured and proximate (co-located) phase shifters.
[0072] The method 400 may further include monitoring 430, for
example by a state control portion of the bi-polar drive circuit, a
device state control signal (e.g. a switching cell control signal)
having a first state and a second state. The method 400 may further
include causing 440 the drive current to flow through (e.g.
substantially only) a first one of the pair of phase shifters when
the device state control signal is in the first state and to flow
through (e.g. substantially only) a second one of the pair of phase
shifters when the device state control signal is in the second
state.
[0073] In some embodiments, the pair of phase shifters are
connected together in reverse-parallel to an output of the drive
circuit. In this case, the causing 440 can include controlling a
polarity of the drive current based on the device state control
signal. The causing 440 can thereby include causing 442 the drive
current to flow through a phase shifter circuit in a first
direction when the device state control signal is in the first
state and to flow through the phase shifter circuit in opposition
to the first direction when the device state control signal is in
the second state. The phase shifter circuit may include a pair of
carrier injection phase shifters (or other asymmetrically
conducting phase shifters) coupled respectively to each of a pair
of waveguide arms of the optical device. The pair of carrier
injection phase shifters may be connected in parallel in opposite
polarities and electrically coupled to an output terminal of the
bi-polar drive circuit. In some embodiments, the causing 440 may
include generating the drive current using at least one current
source of the bi-polar drive circuit. In other embodiments, the
causing 440 may include generating a drive voltage at an output of
the bi-polar drive circuit coupled to both of the pair of phased
shifters, and the drive current may flow in response to the drive
voltage.
[0074] The method 400 may further include operating the level
control portion of the bi-polar drive circuit at a first speed to
provide the analog level setting output signal; and operating the
state control portion of the bi-polar drive circuit at a second
speed, higher than the first speed, to control the polarity of the
drive current.
[0075] When the drive circuit is a unipolar drive circuit, the
causing 440 can include another operation 444 which includes
operatively coupling the drive circuit to a first one of the pair
of phase shifters using a switch to cause the drive current to flow
through that first one of the pair of phase shifters. Similarly,
with respect to the operation 444, causing the drive current to
flow through the second one of the pair of phase shifters can
include operatively coupling the drive circuit to the second one of
the pair of phase shifters using the same switch. The switch may
therefore connect the source of drive current (or a corresponding
source of drive voltage) to a selected one of the pair of phase
shifters at a time.
[0076] Although method 400 for controlling an optical device, such
as an MZI-based push-pull CIOS cell, is discussed herein above, it
will be readily understood that other types of optical devices may
be controlled using the method 400, such as a Micro Ring Resonator
(MRR), an MZI device comprising an MRR, a plurality of MRRs in
series or parallel, an optical switch, an optical attenuator, a
variable optical attenuator, a modulator, or a pilot tone
modulator, provided that device requires a push-pull drive.
[0077] The method 400 may be applied separately for each of a
plurality of optical devices. In this manner, level setting and
device state control for each optical device can be performed
separately. More specifically, separate level setting control
signals are provided for each optical device. To achieve this, each
separate level setting control signal can be associated with a
separate device state control signal.
[0078] FIG. 5 illustrates, in a component view, an example
apparatus 500 for driving a plurality of MZI-based push-pull CIOS
cells in electrical domain according to an embodiment of the
present disclosure. The apparatus 500 comprises a plurality of the
bi-polar drive circuits and a plurality of the MZI-based push-pull
CIOS cells as described herein above. Each one of the bi-polar
drive circuits is electrically coupled to a single MZI-based
push-pull CIOS cell from the plurality of the MZI-based push-pull
CIOS cells. Further, each one of the bi-polar drive circuits may
include one DAC channel to drive a pair of carrier injection phase
shifters as discussed herein above, and thus the number of the
plurality of the bi-polar drive circuits is half of the number of
the carrier injection phase shifters. The plurality of the
high-speed common mode reference node from each of the plurality of
the MZI based optical switching cell may be electrically coupled to
a single DC bias or different DC biases.
[0079] The example embodiment as illustrated in FIG. 5 allows using
only one DAC channel per each bi-polarity switch cell and provides
drive current that can be independently adjusted to each
bi-polarity CIOS cell. This solution provides the CIOS cells very
fast switching of current (or voltage) direction and reduces the
number of DAC required in the driving circuit.
[0080] FIGS. 6A and 6B illustrate a Micro-ring Resonator device
(MRR) 620 comprising a pair of phase shifters D1, D2, which can be
electrically connected and driven according to embodiments of the
present disclosure, for example using the drive circuit 110 of FIG.
1, or its variants as illustrated in FIGS. 2, 3A to 3F, or using
the method as illustrated in FIG. 4. Details of the MRR can be
found for example in "Silicon Micro-ring Resonator Device Design
for Optical Interconnect Systems "Li, Yunchu. PhD Thesis.
University of Southern California, ProQuest Dissertations
Publishing, 2013. The phase shifters D1, D2 of the MRR 620 may be
driven in push-pull mode, and may correspond to (forward-biased PIN
junction) carrier injection phase shifters, for example. The MRR
620 comprises at least a pair of waveguides coupled together using
a pair of optical couplers, wherein one of the waveguides is
configured as a loop or ring. In FIG. 6A, the MRR 620 comprises a
variable (controlled) optical coupler, the optical coupler being
implemented as a Mach-Zehnder device. In FIG. 6B, the MRR 620
comprises a variable (controlled) optical coupler, the optical
coupler being implemented as a ring-assisted Mach-Zehnder device. A
pair of ring devices, which include the phase shifters D1, D2 as a
portion thereof (precise location not shown), control the behavior
of the overall larger MRR device 620. Embodiments of the present
disclosure may comprise one or a plurality of MRR devices 620.
[0081] Additionally, in various embodiments discussed herein above,
multiple ground pads may be provided in order to mitigate
electrical crosstalk. For example, ground pads electrically coupled
to different phase shifters can be separated.
[0082] Control electronics for providing the level setting input
signals and device state control signals, and for managing
calibration operations, can include computer processors operatively
coupled to memory, microcontrollers or other electronic circuitry
as would be readily understood by a worker skilled in the art.
Optical components providing optical signals to and receiving
optical signals from the optical devices described herein will also
be readily understood by a worker skilled in the art, and may
include optical communication devices, optical emission devices and
optical sensing devices.
[0083] Although the present disclosure has been described with
reference to specific features and embodiments thereof, it is
evident that various modifications and combinations can be made
thereto without departing from the disclosure. The specification
and drawings are, accordingly, to be regarded simply as an
illustration of the disclosure as defined by the appended claims,
and are contemplated to cover any and all modifications,
variations, combinations or equivalents that fall within the scope
of the present disclosure.
* * * * *